Influence of the Soil Water Content and Distribution on Both the Hydraulic and Transpiration Performance of ‘Manzanilla’ Olive Trees

This work was made with mature ‘Manzanilla’ olive trees in an orchard of a semi-arid area in southern Spain. Three water treatments were considered: Rainfed, in which the trees had rainfall as the only source of water supply; FAO, in which the trees were under localized irrigation to replace the crop water demand, with some roots left in drying soil; Pond, in which the whole rootzones of the trees were maintained under non-limiting soil water conditions for the whole dry season. Our aim was to obtain information on the mechanisms behind the reduction of transpiration (Ep) in the FAO trees, as compared to the Pond trees. Our results show a near-isohydric behaviour of the FAO trees, i.e. those trees under localized irrigation in which some roots are left in drying showed lower stomatal conductance than the Pond trees in which all roots were in wetted soil. This helped the FAO trees to maintain similar leaf water potentials than the Pond trees. In addition, the FAO trees maintained a constant difference between the water potential of the canopy and that in the soil. This has been described as an isohydrodynamic behaviour, and it is thought to be an improvement over a typically anisohydric behaviour. These mechanisms were behind the similar values of tree hydraulic conductance (Kp) found in the FAO and Pond treatments. The Rainfed trees showed lower Kp values because of the low Ep values of those trees, due to the low soil water availability in that treatment. Our results show, however, that the Rainfed trees were able to maintain similar values of Kp all throughout the dry season, which shows that the hydraulic efficiency of the xylem of those trees was little affected by embolism, despite of the high demanding conditions in the area. INTRODUCTION Evidence shows that both the tree transpiration (Ep) and the production of mature fruit trees under field conditions are influenced not only by the amount of water supplied by irrigation, but also by the distribution of that water in the soil. Fernández et al. (2003) estimated Ep values from sap flow measurements in mature ‘Manzanilla’ olive trees under different irrigation regimes, and reported an Ep increase of 37% when trees under localized irrigation were suddenly irrigated with a pond irrigation that wetted the whole rootzone. This was despite of the trees under localized irrigation being irrigated daily with enough water to replace the crop evapotranspiration (ETc). Pastor and Vega (2005) reported increases in the yield of mature ‘Picual’ olive trees irrigated with an increasing number of drippers, although with the same water amounts. It seems, therefore, that both water consumption and yield performance are reduced in olive trees in which part of their J.E. Fernández and J.M. Torres-Ruiz Instituto de Recursos Naturales y Agrobiología IRNASE-CSIC Apartado 1052, 41080-Sevilla Spain J.L. Muriel and R. Romero IFAPA, Centro Las Torres-Tomejil M.J. Martín-Palomo and A. Morales-Sillero Departamento de Ciencias Agroforestales Universidad de Sevilla 41013-Sevilla Spain A. de Cires and A.E. Rubio-Casal Dpto. de Biología Vegetal y Ecología Ctra. Sevilla-Cazalla, km 12.2 41200-Alcalá del Río, Sevilla Spain Universidad de Sevilla Apdo. 1095, 41080-Sevilla Spain Proc. 6 IS on Irrigation of Hort. Crops Eds.: S. Ortega-Farias and G. Selles Acta Hort. 889, ISHS 2011 324 root system is left under soil drying conditions during the irrigation season. Several mechanisms could be involved in that reduction, including changes on the soil-to-root hydraulic conductance (Tuzet et al., 2003), on the hydraulic efficiency of the xylem (Lo Gullo et al., 2003) and on root-to-shoot signalling mechanisms influencing stomatal closure (Davies et al., 2000). The existing literature for olive on that matter was reported by Fernández et al. (2009a). They outlined the lack of information on these aspects for olive trees growing under field conditions in semi-arid and arid areas. The aim of this work was to evaluate the influence of different soil water regimes on the seasonal trends of stomatal conductance (gs), leaf water status, Ep and hydraulic conductance (Kp) of mature ‘Manzanilla’ olive trees of an orchard located in a semi-arid area of south Spain. The experiments were design to obtain information on the mechanisms controlling the reduction of transpiration found in olive trees under localized irrigation. MATERIALS AND METHODS Orchard Characteristics and Water Treatments The experiments were made during the irrigation season of 2007, in a 0.5 ha olive orchard (37°17’ N, 6°3’ W, 30 m a.s.l.) with 39-year-old ‘Manzanilla de Sevilla’ olive trees at 7×5 m spacing. The trees have a single trunk with two main branches from 0.7 to 1.5 m above ground. Average values of canopy volume and leaf area density at the end of the growing season were 42 m and 1.6 m m, respectively. The soil is a sandy loam of about 1.6-2.0 m depth, depending on the location. The texture is quite homogeneous, with average values of 14.8% clay, 7.0% silt, 4.7% fine sand and 73.5% coarse sand. The climate is typically Mediterranean, with a mild, wet season from October to April; the rest of the year is hot and dry. The total precipitation (P) collected in 2007 was 411.1 mm and the potential evapotranspiration (ETo, FAO56 Penman-Monteith equation) was 1235.0 mm. The orchard was divided into three plots, each with a different water treatment: 1) Rainfed, with rainfall as the only source of water supply; 2) FAO, in which trees were irrigated daily from May 14 to October 2, with enough water to replace ETc. The irrigation system consisted of a lateral per tree row, with five 3 L h drippers per tree, 1 m apart. With this system some roots were left in drying soil. Irrigation doses were calculated with the crop coefficient approach, as described by Fernández et al. (2006a). Basically, ETc was calculated as ETc = Kc Kr ETo, with crop coefficient (Kc) values of 0.76 in May, 0.70 in June, 0.63 in July and August, 0.72 in September and 0.77 in October. The coefficient related to the percentage of ground covered by the crop (Kr) was 0.71; 3) Pond, in which the trees were irrigated with a grid of pipes with a 2 L h dripper every 0.4×0.4 m. The grid covered a surface of 8×6 m, with the tree in the middle, enough to keep non-limiting soil water conditions in the whole rootzone, all throughout the irrigation season. Hydraulic Conductance of the Trees The Kp values were determined as Kp = Ep ΔΨ , being Ep measured at the central hours of the day, when transpiration rates were relatively constant; ΔΨ =s-l, where s (MPa) is the ‘effective’ soil water potential at the root surface and l (MPa) is the ‘effective’ leaf water potential for the whole canopy (Jones, 1983). Boths and l were determined at the same hours than Ep. The values of l were estimated asl =  o + (1)i (Moreshet et al., 1990), where Ψo (MPa) is the leaf water potential of sunlit leaves outside the canopy, Ψi (MPa) is the leaf water potential of shade leaves inside the canopy, and α is the fraction of sunlit and shade leaves (calculated as 0.43 for the orchard conditions by Diaz-Espejo et al., 2002). We assumed that Ψs = Ψl predawn, being Ψl predawn the average value of water potential measured at predawn in leaves sampled from the base of the trunk. The values of Ep were determined from sap flow measurements made with the Tz